1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device implementing in-plane switching (IPS) where an electric field to be applied to a liquid crystal is generated in a plane parallel to a substrate.
2. Description of Related Art
Recently, light and thin liquid crystal display (LCD) devices with low power consumption are used in office automation equipment, video devices, and the like. Such LCD's typically use an optical anisotropy and spontaneous polarization of a liquid crystal (LC). The liquid crystal has a thin and long liquid crystal molecule, which causes a directional alignment of the liquid crystal molecules. At this point, an alignment direction of the liquid crystal molecules is controlled by applying an electric field to the liquid crystal molecules. When the alignment direction of the liquid crystal molecules is properly adjusted, light is refracted along the alignment direction of the liquid crystal molecules to display image data. Of particular interest is an active matrix (AM) LCD, in which a plurality of thin film transistors and pixel electrodes are arranged in the shape of an array matrix, because of its high resolution and superiority in displaying moving pictures. Driving methods for such LCD's typically include a twisted nematic (TN) mode and a super twisted nematic (STN) mode. A TN liquid crystal panel has high transmittance and aperture ratio. In addition, since the common electrode on the upper substrate serves as a ground, static electricity is prevented from destroying the liquid crystal panel.
Although TN LCD's and STN LCD's, which have the same structure, have been put to practical use, they have a drawback in that they have a very narrow viewing angle. In order to avoid the problem of narrow viewing angle, IPS LCD devices have been proposed. IPS LCD devices typically include a lower substrate where a pixel electrode and a common electrode are disposed, an upper substrate having no electrode, and a liquid crystal interposed between the upper and lower substrates. The IPS LCD device has advantages in contrast ratio, gray inversion, and color shift that are related to the viewing angle.
FIG. 1A is a plan view illustrating in detail the structure of one pixel region in an IPS-LCD device, specifically, a unit pixel region 10. In addition, a cross-sectional view taken along a line “B—B” in FIG. 1A is illustrated in FIG. 1B.
On the surface of a lower substrate 1a adjacent to the liquid crystal layer, a scan signal line 2 made of, for example, aluminum (Al) is formed extending along the x-direction, as shown in FIG. 1A. In addition, a reference signal line 4, also known as a common line, is formed extending along the x-direction, close to the scan signal line 2 on the +y-direction side thereof. The reference signal line 4 is also made of, for example, Al. A region surrounded by the scan signal line 2, the reference signal line 4, and the video signal lines 3 constitutes the unit pixel region 10.
In addition, the unit pixel region 10 includes a reference electrode 14 formed by the reference signal line 4 (identified as 4(14)), and another reference electrode 14 formed adjacent to the scan signal line 2. The pair of horizontally extending reference electrodes 14 are positioned adjacent to one of a pair of video signal lines 3 (on the right side of the FIG. 1A), and are electrically connected to each other through a conductive layer 14a, which is formed simultaneously with the reference electrodes 14.
In the structure described above, the reference electrodes 14 form a pair extending in the direction parallel to the scan signal line 2. In other words, the reference electrodes form a strip extending in a direction perpendicular to the video signal lines 3, later described.
A first insulating layer 11 (see FIG. 1B) made of, for example, silicon nitride is formed on the surface of the lower substrate 1a on which the scan signal lines 2 are formed, overlying the scan signal line 2, the reference signal lines 4, and the reference electrodes 14. The first insulating layer 11 functions as an inter-layer insulating film for insulating the scan signal line 2 and the reference signal line 4 from the video signal lines 3, (b) as a gate-insulating layer for a region in which a thin film transistor (TFT) is formed, and (c) as a dielectric film for a region in which a capacitor “Cstg” is formed. The TFT includes a drain electrode 3a and a source electrode 15a. A semiconductor layer 12 for the TFT is formed near a crossing point of the scan signal line 2 and video signal line 3. A first polarization layer 18 is formed on the other surface of the lower substrate 1a. 
On the first insulating layer 11, a display electrode 15 is formed parallel with the reference electrode 14. One end portion of the display electrode 15 is electrically connected to the conductive layer 14a, and the other end portion thereof is electrically connected to the source electrode 15a. Still on the first insulating layer 11, a first planar layer 16 is formed to cover the display electrode 15. A first alignment layer 17 is formed on the first planar layer 16.
FIG. 1B also illustrates a cross-sectional view of the upper substrate 1b on which a black matrix 300 is formed. A color filter 25 is formed to close an opening in the black matrix 300. Then, a second planar layer 27 is formed to cover the color filter 25 and the black matrix 300. A second alignment layer 28 is formed on the surface of the second planar layer 27 facing the liquid crystal layer.
The color filter 25 is formed to define three unit pixel regions adjacent to and extending along the video signal line 3 and to position a red (R) filter, a green (G) filter, and a blue (B) filter, for example, from the top of the three unit pixel regions. The three unit pixel regions constitute one pixel region for color display.
A second polarization layer 29 is also arranged on the surface of the upper substrate 1b that is opposite to the surface of the upper substrate 1b adjacent to the liquid crystal layer, on which various layers are formed as described above.
It will be understood that in FIG. 1B, a voltage applied between the reference electrodes 14 and the display electrode 15 causes an electric field “E” to be generated in the liquid crystal layer “LC” in parallel with the respective surfaces of the lower and upper substrates 1a and 1b. This is why the illustrated structure is referred to as the in plane switching, as mentioned above.
With reference to FIGS. 2, 3A, and 3B, operation modes of a typical IPS LCD device are explained in detail.
FIG. 2 is a conceptual cross-sectional view illustrating a typical IPS LCD device. As shown, lower and upper substrates 1a and 1b are spaced apart from each other, and a liquid crystal “LC” is interposed therebetween. The lower and upper substrates 1a and 1b are called an array substrate and a color filter substrate, respectively. On the lower substrate 1a, pixel and common electrodes 15 and 14 are disposed. The display and reference electrodes 15 and 14 are positioned parallel with and spaced apart from each other. On a surface of the upper substrate 1b, a color filter 25 is disposed opposing the lower substrate 1a. The display and reference electrodes 15 and 14 apply an electric field “E” to the liquid crystal “LC”. The liquid crystal “LC” has a negative dielectric anisotropy, and thus it is aligned parallel to the electric field “E”. The display electrode 15 and reference electrode 14 are also referred to as the pixel electrode 15 and common electrode 14.
FIGS. 3A and 3B illustrate operation modes for the typical IPS-LCD device shown in FIG. 2. For an off state, the long axes of the liquid crystal molecules “LC” maintain some angle with respect to an invisible line that is perpendicular to the pixel and common electrodes 15 and 14. The angle is 45 degrees, for example. At this point, the pixel and common electrodes 15 and 14 are parallel with each other.
For an on state, an in-plane electric field “E”, which is parallel to the surface of the lower substrate 1a, is generated between the pixel and common electrodes 15 and 14. The reason is that the pixel electrode 15 and common electrode 14 are formed together on the lower substrate 1a. Then, the liquid crystal molecules “LC” are twisted such that the long axes thereof coincide with the electric field direction. Thereby, the liquid crystal molecules “LC” are aligned such that the long axes thereof are perpendicular to the pixel and common electrodes 15 and 14. The liquid crystal used in the above-mentioned IPS LCD panel includes a negative dielectric anisotropy.
Returning to FIG. 1B, the second alignment layer 28 is formed on the second planar layer 27. The second planar layer 27 is an overcoat made of acrylate-based or epoxy-based resin and serves to protect the color filter 25. In addition, the second planar layer 27 compensates for stepped portions due to the color filter 25 such that a platen surface is provided for the upper substrate 1b having the color filter 25. After the second planar layer 27 and second alignment layer 28 are sequentially formed on the color filter 25, the second alignment layer 28 is rubbed using a rubbing roller. Then, the second alignment layer 28 has an alignment direction for “off state” of the liquid crystal molecules “LC” shown in FIG. 4A. At this point, because the second planar layer 27 is relatively soft, it is stained during the above-mentioned rubbing step such that an abnormal line pattern is formed thereon. If the alignment layer is stained, the liquid crystal molecules are abnormally aligned in off state.
For the foregoing reason, there is a need for an IPS LCD device that prevents the above-mentioned stained error of the alignment layer.